Die for forming lobed threads

ABSTRACT

Thread rolling die having grooves and ridges which are undulated with respect to the place of die movement with the spacing of the undulations in the direction of die movement being related mathematically to the pitch and major diameters of a screw formed by the die.

United States Patent I [1 1 Simons et al.

DIE FOR FORMING LOBED THREADS Inventors: Leon Simons, New City, N.Y.;

Samuel H. Lessig, Jr., Statesville, -N.C.; Richard H. Corrette,

()lmstead Falls, ()hio Assignee: NL Industries Inc., New York, NY.

Filed: Dec. 11, 1972 Appl. N0.: 313,845

[1.8. CI. 72/469 Int. Cl B21h 3/06 Field of Search 72/469, 88, 90; 151/92; 85/46; 10/10, 152 R, 152 T References Cited UNITED STATES PATENTS 7/1944 Tomalis 151/22 UX [451 Apr. 2, 1974 2,484,645 10/1949 Baumle 151/22 3,163,196 12/1964 Hanneman 151/37 Primary Examiner-Milton S. Mehr Atlnrney, Agent, or Firm-Fitzpatrick, Cclla, Harper & Sclnto [57] ABSTRACT Thread rolling die having grooves and ridges which are undulated with respect to the place of die movement with the spacing of the undulations in the direction of die movement being related mathematically to the pitch and major diameters of a screw formed by the die.

12 Claims, 6 Drawing Figures DIE FOR FORMING LOBED THREADS This invention relates to the rolling of screw threads and more particularly it concerns an improved thread rolling die for producing lobed thread formations on self tapping screws.

In U.S. Pat. applications Ser. No. 54,716 filed July 14, 1970 and US. Pat. Ser. No. 251,023 filed May 8, 1972, there is shown and described a novel self tapping screw having thread formations which are spirally lobed to permit the screw to be turned into an unthreaded hole and form its own thread therein by a swaging action. In a related U.S. Pat. application Ser.

No. 7,280 filed Jan. 30, 1970, there is shown a thread rolling die for producing, from blanks having circular cross section, screws with lobed thread configurations. As shown in that application, the thread rolling die comprises a block of hardened metal having a working face thereof formed with alternate grooves and ridges. The cross section of these grooves and ridges corresponds to the cross section of screw threads produced by a pair of the dies when a blank is rolled between them. The threads are given a lobular formation by means of pressings in the dies. These pressings are undulations along the length of the grooves and ridges with respect to the plane of the working face of the die. In general, three such undulations are produced along a length corresponding to the circumference of the screw blank being formed so that a trilobular thread will be produced. Different numbers of lobes may be produced, however, by changing the shape and spacing of the undulations or pressings. Also, the undulations, as shown, may be arranged with respect to each other in adjacent grooves and ridges such that when impressed into the screw being rolled, the resulting lobes on the screw will spiral around its circumference from one end of the screw to the other.

The present invention is directed to novel thread roll- I ing die arrangements which are capable of impressing accurately contoured lobular formations on threaded screws with minimal distortion of the thread spiral and with minimal distortion of the thread cross section. Past attempts to impress lobed configuration on threaded scribed difficulties by means of a thread rolling die arrangement in which certain dimensional relationships are maintained. The present invention is based in part on the discovery that even though lobe spacing on a .screw is normally measured along the screw thread, the critical dimensional component of corresponding undulation spacing along a thread rolling die is not in 'a direction along the grooves and ridges of the die but rather it is in the direction of relative movement of the die, i.e., perpendicular to the axis of the thread being rolled. The present invention is based in further part on the discovery that when lobular threads are rolled into a cylindrical member by means of a die having lobe forming pressings or undulations, the undulation spacing on the die does not correspond to any one of the blank diameter, the pitch diameter or the major diameter of the threaded member. Instead, the die undulation or pressing spacing corresponds to a cylindrical surface having a diameter somewhere intermediate the pitch and majordiarnetersof the threaded member More specifically, it has been found that the spacing between successive lobe forming pressings on a die, measured in the direction of die rolling movement, i.e., perpendicular to the axis of the screw being rolled, will be found on the screw where it intersects a cylindrical surface coaxial therewith, the cylindrical surface having a diameter approximately half way between the screw pitch diameter and the screw major diameter.

The significance of the foregoing discoveries lies in the fact that they make possible the design of a lobe forming die which will roll lobular threads into a member without danger of thread distortion, waviness or thelike. This is done, according to the present invention, by spacing the lobe forming undulations along a die, in the direction of die movement, such that the distance between the corresponding portions of successive lobe forming undulations or pressings is related to the major diameter and pitch diameter of the screw being formed. According to this relationship, the lobe spacing equals the circumferential distance about a cylinder having a diameter equal to the pitch diameter plus 55 percent of the difference between the pitch and major diameters when that distance is divided by the member of lobes to be formed in one revolution about the cylinder.

A specific embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawing, forming a part of the specification, wherein:

FIG. 1 is a side view of a lobed, threaded screw of the type which may be rolled using the present inventron;

FIG. 2 is a perspective view showing a thread rolling arrangement in which the present invention is embodied;

FIG. 3 is an enlarged elevational view of a die used in the arrangement of FIG. 2;

FIG. 4 is an enlarged fragmentary perspective view showing an undulated portion of the die of FIG. 3;

FIG. 5 is an enlarged fragmentary section view taken along line 55 of FIG. 3; and

FIG. 6 is an enlarged stylized view illustrating a rolling operation according to the present invention.

The screw shown in FIG. 1 comprises a hexagonal head 10 and a threaded shank 12. The shank 12 is formed with threads 14 which as shown in the drawing are of standard 60 V contour. Actually, the specific form of thread is not especially important to the present invention, although it is contemplated that the invention will find greatest application to the formation of threads corresponding to the Unified Screw Thread Standard or to the American National screw thread system.

The threads 14 in the machine screw of FIG. 1 do not extend smoothly about the screw at a fixed distance from the screw axis, but instead they undulate up and down with respect to the pitch circumference of the screw. These undulations form lobes cross sectionally of the screw and in the present case three such lobes occur for each revolution of the thread 14. The lobes, moreover, are arranged spirally about the screw and this spiral, which is indicated by lines 16 separating adjacent lobes, extends in a direction opposite to that of the thread helix and has a pitch or lead considerably greater than that of the thread helix.

The lobed thread 14 is the same as described in copending US. Pat. application Ser. No. 54,716, filed July 14, 1970, and assigned to the assignee of the present invention.

As shown in FIG. 2, the lobed thread 14 is formed, according to the present invention, by rolling a cylindrical or slightly conical blank 18 between a pair of special rolling dies 20 and 21. The dies 20 and 21 are mounted in a machine 22 which holds them in parallel alignment a fixed distance apart and causes one or both of the dies to undergo movement with respect to each other along parallel paths as indicated by arrows A and B.

As the dies 20 and 21 approach one another, the blank 18, which may have been previously headed, is inserted in the space between the dies. Continued relative movement of the dies in the direction of the arrows A and B causes the blank 18 to roll between them as indicated by an arrow C.

It will be noted that the dies 20 have mutually facing thread forming surfaces 24 which are formed with ridges 26 and grooves 28 having cross sectional con tours corresponding to those of screw threads to be formed. Also, the ridges and grooves are angled slightly downwardly with respect to the paths of relative die movement. With this arrangement, the grooves and ridges operate to press against the outer surface of the blank 18 and to impress into that surface a screw thread formation.

The thread rolling arrangement as thus far described is similar to conventional thread rolling arrangements well known in the art.

As can be seen in FIG. 3, the thread forming surface 24 ofthe die 20 has ridges and grooves 26 and 28 which extend continuously toward the right or entrance end of the die. Toward the left or finishing end of the die, however, the ridges and grooves making up the thread forming surface 24 are interrupted or undulated with respect to the plane of the die working surface. These undulations are arranged in slanted columns as indicated by interruption lines 30. It will be noted that the columns extend at a column angle B with respect to the upper and lower surfaces of the die and with respect to the path of die movement as indicated by the arrow B. This angle B is the same as the helix angle of the lobe spiral shown by the lines 16 in FIG. 1. Also, it will be noted that the ridges and grooves 26 and 28 extend at a helix angle a with respect to the upper and lower die surfaces and path of die movement. This angle a which is in a direction opposite to that of the angle B is the same as the helix angle of the thread 14 in FIG. 1.

Turning now to FIG. 4, it will be seen that the undulations in the forming surface 24 of the dies are actually a series of pressings 32 whereby the ridges and grooves 26 and 28 are actually tilted or slanted slightly with respect to the plane ofthe forming surface without appreciable change in cross section, contour or helix angle a. The depth of the pressings 32, i.e., the distance from the highest to the lowest point on the ridge or on the groove of each pressing measured in a direction perpendicular to the plane of the working surface 24, may vary according to thread size. In general, the pressing depth extends from 0.003 inches for a No. 2-64 fine thread screw to 0.0l inches for a 36-16 coarse thread screw. It will be noted that each pressing 32 is asymmetrical. That is, it rises gradually outwardly with respect to the plane of the working surface over most of its length in a leftward direction and then returns rather abruptly back inwardly with respect to the working surface plane. This configuration produces an asymmetrically lobed thread which has been found to be of particular significance in the manufacture of thread forming screws.

It will be understood that the die 21 has thread forming ridges and grooves and lobe forming undulations or pressings identical in size and shape to those of the die 20, but reversed as to column and helix angle B and a so as to complement the die 20. Thus, the formations on the working surface of the die 21 appear to be continuations of the formations on the die 20 to the surface of a blank after it has rolled from one die surface to the other.

In order to avoid the formation of double threads or distorted threads in rolling operations, it is important that the helix angle a of the two dies 20 and 21 be identical. Also, the relative vertical positioning, i.e., along the axis of the rolled blank, of the two dies must be precisely controlled. Otherwise, the ridges and grooves in one die will not appear, to the blank being rolled, as continuations of the ridges and grooves of the other die; and each die then will attempt to form its own set of threads.

A corresponding criticality exists with respect to the positioning of the pressings 32 in the finishing region of the two dies. The undulations on one die must complement those of the other die so that the lobe formations produced in the blank by the pressings of one die will register properly with pressings of the other die.

It has further been found that the spacing of the pressings 32 must be properly controlled in order to produce a properly formed screw with lobed threads. Thus, if the spacings between the pressings 32 are too short, the lobes will be distorted and in fact double lobes or extra lobes will result. On the other hand, if the spacings are too long, the thread contour will be affected and a wavy or drunken thread will result. The spacing of the pressings as used herein, means the distance between corresponding points on successive die pressings along the direction of relative die movement.

One of the important discoveries underlying the present invention is the relationship of pressing spacing to screw size and its effect on lobe and thread configuration Thus, it has been found that the pressing spacing should not be related in direct proportion to the size, i.e., the circumference, of the screw to be rolled. Rather, it should be related to both the pitch diameter and the thread depth of the screw, according to a particular function. The function which relates the pressing spacing to these two parameters of pitch diameter and thread depth is as follows:

L PD R[MD PD] (vr/N) where:

L pressing spacing in the direction of relative die movement, i.e., perpendicular to the thread rolling axis (see FIG. 3);

PD the pitch diameter of the thread being'rolled;

MD the major diameter of the thread being rolled;

R= a multiplication factor. It has been found that optimum results are obtained when R is equal to 0.55. However, acceptable results may be expected when R is within the range of 0.30 to 1.00;

11' pi," i.e. 3.]416; and

N the number of lobes to be formed in one revolution of the thread being rolled.

It will be understood that the pressing spacing along a die is not necessarily equal to pressing length. Pressing length as used herein means the actual length of a pressing measured along the ridge or groove on which the pressing occurs. Since the ridges and grooves are at an angle a with respect to the direction of relative die movement, it will be appreciated that it is possible for the pressings individually to have a length greater than their pressing spacing." On the other hand, the pressing length may be shorter, for example, where it is desired to form lobes which are not immediately adjacent to each other. Also, where the lobes are to be spiralled around the screw to be rolled, the columns of pressings are slanted at an angle B which effectively increases the length of the pressings beyond the pressing spacing distance.

It will be understood that in order to obtain proper registry between the formed screw thread lobes and the die pressings during rolling, each pressing on the die must be spaced from a corresponding second pressing, in the direction of relative die movement, by an amount such that a lobe formed on a screw by the first pressing will thereafter register with the corresponding second pressing when the screw has been rolled 360. However, because of the thread spiral on the screw and the groove and ridge angle, a, on the die, the second or corresponding pressing will be located along a ridge and groove which is adjacent, i.e., either above or below, the ridge and groove on which the first pressing is formed. Thus, irrespective of the number of lobes to be formed per screw revolution, irrespective of the screw thread angle and irrespective of the lobe spiral, the

pressings on the die must be spaced such that corresponding points of successive pressings located along the direction of relative die movement and in mutually adjacent grooves and ridges are displaced from each other by an amount equal to: 4

[PD R(MD PD)]1r where PD pitch diameter of the screw;

R a range from 0.30 to 1.00;

MD major diameter of the screw; and

1r pi (3.1416).

The complex interaction of forces and various other factors which take place during the rolling of a lobed thread has not been fully analyzed, so that a complete presentation of the theory underlying the present invention has not as yet been developed. However, it is believed that the invention functions on the basis that during the rolling of a screw thread, the point of actual rolling does not occur at the root, the pitch line or even at the crest of the threads being rolled. Instead, it is believed that the point of actual rolling occurs somewhere between the pitch line and the crest of the threads being formed, and more likely, it is close to 55 percent of the distance from the pitch line to the crest.

The above may be visualized more easily by a consideration of FIG. 6. -As there shown in stylized form in cross section, a trilobular threaded blank 40 is rolled between two threaded dies 42 and 43. It should be understood that FIG. 6 does not accurately represent actual conditions. It will be seen in the drawing that the threaded blank is shown as having a thread crest 44 and a thread root 46, both represented in solid outline. Similarly, the drawing shows the dies 42 and 43 as having thread forming ridges 48 and grooves 50, also represented in solid outline. In a true cross section the thread roots 46 of the blank 40 and the grooves 50 of the dies 42 and 43 would be shown in dotted outline. Also in an actual cross section the thread on the blank 40 would spiral down and become overlapped on itself at some point and the lobes also would experience a similar condition since they also undergo spiraling. The discrepancies described above have been introduced to simplify the drawing and to aid in visualizing the rolling action of the blank 40 between the dies.

During rolling of the blank 40, the upper die 42 moves rightwardly as indicated by an arrow D while the lower die 43 remains stationary. This, of course, corresponds to relative leftward movement of the lower die 43 with respect to the upper die 42, as indicated by the arrow E. This relative die movement causes the blank 40 to roll clockwise about an axis 0 as indicated by an arrow F. During this rolling the blank thread crest 44 contacts the die grooves 50 and the blank thread root 46 contacts the die ridges 48. Now the dies 42 and 43 remain a fixed distance apart and the axis 0 of the blank 40 remains centered between them. Thus, the rolling of the blank thread on either die resembles the rolling of a flanged wheel on a track. If the point of roll against the lower die 43 is at the point of contact between the blank thread root 46 and the corresponding die ridge 48, then the thread crest 44 must slip backwardly relative to the corresponding die groove 50 as indicated by the arrow G. On the other hand, if the point of roll is at the thread crest 44 of the blank (and at the bottom of the disc groove 50), then the thread root 46 is forced to slip forwardly as indicated by the arrow H. Similar phenomenon are experienced where the blank 40 engages the upper die 42. For any point of rolling between the crest and root, smaller slippages will occur at both the crest and root. Because of these slippages, a given distance along a die in any plane other than that in which actual rolling takes place does not correspond to the same distance along the thread being rolled. While this does not have any appreciable effect upon the formation of a continuous thread, it does have an effect where irregularities, i.e., lobes, are to be formed by the disc. In the illustrated case, where the die formations have pressings 52, the spacing L between successive pressings, will not register with corresponding lobe formations 54 on the blank during rolling unless than distance is the same as the circumferential distance about the blank 40 measured at the plane of actual roll. This point of actual roll appears to be somewhere between the pitch diameter and major or crest diameter of the thread being rolled and more nearly half way between the two. Thus, accord ng to the present invention, the distance between successive lobe forming pressings along a die is chosen to be equal to the circumferential distance between successive lobes to be formed on the screw measured along a cylindrical plane having a diameter approximately half way between the pitch diameter and the crest or major diameter of the screw.

It will be appreciated that although the formula discussed herein relates pressing distance on the die to the pitch and major diameters of something outside the die, i.e., the screw being rolled, nevertheless the formula can be applied to any standard thread rolling die. This is because the thread angle a, the relative depth of the 7 8 grooves to the ridges and the number of grooves and depths of the grooves with respect to the ridges, their ridges along the roll axis of any die establishes a speangle with respect to said given line and the number of cific pitch diameter and major diameter of a thread to said ridges and grooves along a second line in said be rolled. Thus, the pitch diameter (PD) to which a 'plane perpendicular to said given line together estabparticular die will roll can be calculated by the followlishing a pitch diameter and a major diameter of a ing formula: thread to be formed by rolling a cylindrical blank over said one surface in the direction of said given line with PD cot 01/77 T the axis of said blank parallel to said second line, said where: ridges and grooves being undulated with respect to the a the angle of the grooves and ridges with respect 10 plane of said one surface over at least a portion of their to the direction of rolling of relative (he move extent to produce a lobular thread on said cylindrical men! measured along a Plane in which the thick blank, the distance between corresponding points of Hess 0f the ridges equals the Space between them the undulations along said given path and in adjacent (i.e., the pitch diameter plane shown in FIG. 5); grooves and ridges is equal to and T the number of ridges (threads) per inch along a R(MD line perpendicular to the direction of rolling. Where; The major diameter (MD) to which a particular die PD pitch diameter f said Screw; will roll can be calculated by the following formula: 5

MD PD 2d MD major diameter of said screw; and

Tr pi (3.1416). where: 1

d the distance from the pitch diameter plane to the 3. A thread lobe rolling die comprising a bioglg havbottoms of the die grooves. I ing a working surface formed with adjacent parallel Having thus described the invention with particular grooves and ridges corresponding in cross section to reference to the preferred form thereof, it will be obvithe cross sectional configuration of screw threads, said oils to those skilled in the art to which the invention block having mounting arrangements for confining relpertains, after understanding the invention, that variti movement 0f.said ki Surface i i plane ous changes and modifications may be made therein along a given li with respect t a corresponding Without departing from the Spirit and Scope of the working surface of an associated block, the relative vention. as defined by the Claims pp n hefelO- depths of the grooves with respect to the ridges, their e Claiml angle with respect to said given line and the number of A thread lobe forming (he arrangement for said ridges and grooves along a second line in said Pressing lobular Configurations the thread formaplane perpendicular to said given line together estabtions ofa screw by rolling thereof, said die arrangement fi hi a pitch diameter and a major diameter f a Comprising at least two elongated die elements having thread to be formed by rolling a cylindrical blank over mutually facing Working Surfaces, Said Working said one surface in the direction of said given line with faces having thread rolling grooves and ridges formed the axis of said blank parallel to said second line, said therein and said die elements being mounted for rela- 40 ridges and grooves being undulated with respect to the five movement of Said Working Surfaces along a given plane of said one surface over at least a portion of their P for rolling a Screw of a given pitch diameter there extent to produce a lobular thread on said cylindrical between, the thread rolling grooves and ridges on each blank, the distance between corresponding points of of said working surfaces having pressings formed along adjacent undulations along said given line being equai their length with respect to the plane of the working to surface to impress lobes in the threads ofa screw rolled 5 between the die elements, said pressings being configured and distributed along said grooves and ridges such [PD R(MD (n/N) that corresponding points of the pressings along said given path and in adjacent grooves and ridges are displaced from each other by an amount equal to:

where: r PD said pitch diameter of a thread to be formed;

[PD+R(MDPD)]1r 55;

i MD said major diameter of a thread to be formed; where: I 5 i5tch diameter of said screw; v 77 p (11416); and M d N 32 any number of lobes to be formed in one thread;

MD major diameter of said screw; and

1r pi (3.1416). lev lutlon.

A thread lobe rolling die comprising a block havig g gi g formmg accordmg clam} 1 ing a working surface formed with adjacent parallel r r grooves and ridges corresponding in cross section to A thread lobe fofmlhg dle aeeol'dlhg 111 3 wherein said undulations are arranged in columns the cross sectional configuration of screw threads, said block having mounting arrangements for confining rel- 3 g d W rking ia. 7 i ative movement of said working surface in its plane 6. A thread lobe forming die according toglaim 5 along a given line with respect to a corresponding wherein said columns are slanted to form an acute working surface of an associated block, the relative angle with respect to said given line thereby to form lobes which are spirally arranged about a screw formed by said die.

7. A thread lobe forming die according to claim 6 1 wherein the direction of slant of said columns is opposite to that of said grooves and ridges with respect to said given line thereby to form a lobe arrangement which spirals in a direction opposite to the threads of a screw formed by said die.

8. A thread lobe forming die according to claim 3 wherein said undulations are assymetrical thereby to form assymetrical lobes.

wherein said die is configured to form a standard screw having a size between No. 2-64 and %l 6 and wherein the overall depth of said undulations along a given ridge is between 0.003 and 0.010 inches.

10. A fhraaioseramfig die comprising a block hav'-" ing a working surface formed with adjacent parallel grooves and ridges corresponding in cross section to the cross sectional configuration of screw threads, said block having mounting arrangements for confining relative movement of said working surface in its plane along a given line with respect to a corresponding working surface of an associated block, the relative depths of the grooves with respect to the ridges, their angle with respect to said given line and the number of said ridges and grooves along a second line in said where:

PD said pitch diameter of a thread to be formed;

MD said major diameter of a thread to be formed;

Tr=pi(3.14l6); and N any number of lobes to be formed in one thread revolution.

11. A thread lobe forming die according to claim 10 wherein R =0.55. v 1 g 12. A thread lobe forming die according to claim 10 wherein N 3.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- April 2, 1974 Patent No. 300,585 Dated *Inventofls) E Nv SIMONS, ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 49, "than" to,read that;

Column 8, line 19, "0.55+" to read --O.55--;

Column 8, line 57, "N32" to read -.N=

Signed and sealed this 30th day of July 19 74.

(SEAL) I Attest: I

MCCOY M. GIBSON JR. 0. MARSHALL DANN Commissioner of Patents Attesting' Officer UScOMM-DC GOa7c-P69 n u.s. aovulmnn nmmue OH'ICI: nu o-su au, w

FORM PO-IOSO (10-69) 

1. A thread lobe forming die arrangement for impressing lobular configurations on the thread formations of a screw by rolling thereof, said die arrangement comprising at least two elongated die elements having mutually facing working surfaces, said working surfaces having thread rolling grooves and ridges formed therein and said die elements being mounted for relative movement of said working surfaces along a given path for rolling a screw of a given pitch diameter therebetween, the thread rolling grooves and ridges on each of said working surfaces having pressings formed along their length with respect to the plane of the working surface to impress lobes in the threads of a screw rolled between the die elements, said pressings being configured and distributed along said grooves and ridges such that corresponding points of the pressings along said given path and in adjacent grooves and ridges are displaced from each othEr by an amount equal to: (PD + R(MD - PD)) pi where: PD pitch diameter of said screw; R 0.55 MD major diameter of said screw; and pi pi (3.1416).
 2. A thread lobe rolling die comprising a block having a working surface formed with adjacent parallel grooves and ridges corresponding in cross section to the cross sectional configuration of screw threads, said block having mounting arrangements for confining relative movement of said working surface in its plane along a given line with respect to a corresponding working surface of an associated block, the relative depths of the grooves with respect to the ridges, their angle with respect to said given line and the number of said ridges and grooves along a second line in said plane perpendicular to said given line together establishing a pitch diameter and a major diameter of a thread to be formed by rolling a cylindrical blank over said one surface in the direction of said given line with the axis of said blank parallel to said second line, said ridges and grooves being undulated with respect to the plane of said one surface over at least a portion of their extent to produce a lobular thread on said cylindrical blank, the distance between corresponding points of the undulations along said given path and in adjacent grooves and ridges is equal to (PD + R(MD - PD)) pi where: PD pitch diameter of said screw; R 0.55 MD major diameter of said screw; and pi pi (3.1416).
 3. A thread lobe rolling die comprising a block having a working surface formed with adjacent parallel grooves and ridges corresponding in cross section to the cross sectional configuration of screw threads, said block having mounting arrangements for confining relative movement of said working surface in its plane along a given line with respect to a corresponding working surface of an associated block, the relative depths of the grooves with respect to the ridges, their angle with respect to said given line and the number of said ridges and grooves along a second line in said plane perpendicular to said given line together establishing a pitch diameter and a major diameter of a thread to be formed by rolling a cylindrical blank over said one surface in the direction of said given line with the axis of said blank parallel to said second line, said ridges and grooves being undulated with respect to the plane of said one surface over at least a portion of their extent to produce a lobular thread on said cylindrical blank, the distance between corresponding points of adjacent undulations along said given line being equal to (PD + R(MD - PD)) ( pi /N); where: PD said pitch diameter of a thread to be formed; R 0.55 MD said major diameter of a thread to be formed; pi pi (3.1416); and N 32 any number of lobes to be formed in one thread revolution.
 4. A thread lobe forming die according to claim 3 wherein N
 3. 5. A thread lobe forming die according to claim 3 wherein said undulations are arranged in columns along said working surface.
 6. A thread lobe forming die according to claim 5 wherein said columns are slanted to form an acute angle with respect to said given line thereby to form lobes which are spirally arranged about a screw formed by said die.
 7. A thread lobe forming die according to claim 6 wherein the direction of slant of said columns is opposite to that of said grooves and ridges with respect to said given line thereby to form a lobe arrangement which spirals in a direction opposite to the threads of a screw formed by said die.
 8. A thread lobe forming die according to claim 3 wherein said undulations are assymetrical thereby to form assymetrical lobes.
 9. A thread lobe forming die according to claim 3 wherein said die is configured to form a standard screw Having a size between No. 2-64 and 3/8 -16 and wherein the overall depth of said undulations along a given ridge is between 0.003 and 0.010 inches.
 10. A thread lobe rolling die comprising a block having a working surface formed with adjacent parallel grooves and ridges corresponding in cross section to the cross sectional configuration of screw threads, said block having mounting arrangements for confining relative movement of said working surface in its plane along a given line with respect to a corresponding working surface of an associated block, the relative depths of the grooves with respect to the ridges, their angle with respect to said given line and the number of said ridges and grooves along a second line in said plane perpendicular to said given line together establishing a pitch diameter and a major diameter of a thread to be formed by rolling a cylindrical blank over said one surface in the direction of said given line with the axis of said blank parallel to said second line, said ridges and grooves being undulated with respect to the plane of said one surface over at least a portion of their extent to produce a lobular thread on said cylindrical blank, the undulations each having a length, measured along said given line equal to (PD + R(MD - PD)) ( pi /N); where: PD said pitch diameter of a thread to be formed; R 0.55 MD said major diameter of a thread to be formed; pi pi (3.1416); and N any number of lobes to be formed in one thread revolution.
 11. A thread lobe forming die according to claim 10 wherein R 0.55.
 12. A thread lobe forming die according to claim 10 wherein N
 3. 